How do Actinomyces and Streptococcus interact during initial plaque formation?

• A. They compete to the exclusion of the other.
• B. Streptococcus alone establishes attachment; Actinomyces plays no role.
• C. They can competitively and cooperatively bind to the pellicle; Actinomyces helps establish early attachment while Streptococcus species dominate early biofilm dynamics.
• D. They only interact after the biofilm matures.

Answer: (C)

The key idea is that early dental plaque forms through both attachment to the tooth surface and interactions between early colonizers. The tooth surface is covered by the acquired pellicle, a protein-rich film that provides binding sites for bacteria. Streptococcus species are among the first to latch onto this pellicle because their adhesins recognize pellicle components, establishing a base layer. Actinomyces can also attach to the pellicle and, importantly, can coaggregate with streptococci, creating a bridge between the surface and other bacteria. This means the two types of bacteria can compete for available binding sites on the pellicle, yet they also cooperate to form a more stable initial biofilm. Actinomyces helps secure the early attachment, while Streptococcus species quickly populate and shape the early biofilm dynamics. As the community develops, other species join, but the initial setup relies on this blend of competitive and cooperative binding between Actinomyces and Streptococcus.